1. Field of Invention
The present invention relates to an operational amplifier adapting to a source driver, and more particularly, to an operational amplifier that reduces open loop parasitic capacitor and compensate phase margin at relative less components.
2. Description of the Related Art
With great advance in the techniques of electro-optical and semiconductor devices, flat panel displays, such as liquid crystal displays (LCD), have enjoyed burgeoning development and flourished in recent year. Due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. The source driver is an important element of the LCD, which converts a digital data signal used for displaying an image to an analog signal, and outputs the analog signal to each pixel of the display panel. Generally, the source driver has a plurality of driving channels transmitting the analog signal to the pixel in each data line and has a plurality of operational amplifiers for enhancing signal transmission intensity.
FIG. 1 is a circuit diagram of a conventional operational amplifier. Referring to FIG. 1, the operational amplifier 100 includes a non-inverse terminal, an inverse terminal and an output terminal denoted as the nodes A, B and C, respectively. The operational amplifier 100 is a rail-to-rail operational amplifier, which includes the P-type differential pair 110 and the N-type differential pair 120 for increasing the input voltage range that operational amplifier 100 can operate, and avoiding the differential input signal attenuating. The operational amplifier 100 also includes two current mirror circuits 130 and 140 for providing the bias currents to the differential pairs 110 and 120, wherein the P-type transistor P3 and N-type transistor N3 can be seen as current sources driving the current mirror circuits 130 and 140, and the P-type transistors P8 and P9 and N-type transistors N8 and N9 control the bias currents. Besides, the operational amplifier 100 includes the output circuit 150 for generating the output signal to the pixel on display panel.
The P-type transistors P6 and P7 included in the current mirror circuit 130 and the N-type transistors N6 and N7 included in the current mirror circuit 140 are utilized to increase the gain of the operational amplifier 110 and provide the bias currents, but these transistors will produce parasitic capacitor in the circuit of operational amplifier 100. The parasitic capacitor causes the operation of the operational amplifier 100 instable so that the capacitors C1 and C2 coupled to the output circuit 150, which are referred as Miller capacitors, are utilized for compensating the phase margin of the operational amplifier 100. Since the capacitors C1 and C2 may decrease the slew rate of the operational amplified, the bias currents of the operational amplifier 100 are increased for increasing the slew rate, but it would cause power dissipation. In addition, the capacitors C1 and C2 considerably occupy layout area of the operational amplifier 100.